From the viewpoint of global environmental conservation, a constant and important issue in the automotive industry is to improve the fuel consumption of automobiles by reducing the weight of their bodies while the strength of the bodies is maintained in order to reduce CO2 emissions. To achieve a reduction in the weight of automobile bodies while their strength is maintained, it is effective to increase the strength of steel sheets used as the materials of automobile parts to thereby allow the thickness of the steel sheets to be reduced. Many automobile parts made of steel sheets are formed by press forming, burring, etc. Therefore, high-strength steel sheets used as the materials of automobile parts are required to have, in addition to desired strength, high workability. Particularly, ultra-high-strength steel sheets having a tensile strength (TS) of 1,300 MPa or more are required to have, in view of ductility, excellent elongation characteristics (uniform elongation and local elongation). Moreover, high-strength hot-dip galvanized steel sheets are expected as steel sheets with excellent corrosion resistance. In view of the above circumstances, various high-strength steel sheets with excellent workability have been developed.
To increase the strength of a steel sheet, a large amount of alloy elements is added to the steel. This, however, presents a problem in that its manufacturability is impaired and deterioration in quality occurs such as a defective shape and in-plane non-uniformity of material properties, so that sufficient material performance cannot be provided. It is therefore very important to solve the above problem in a comprehensive manner.
As a technique for a high-strength steel sheet having excellent formability, Patent Literature 1 discloses a technique for a high-strength cold-rolled steel sheet having high strength, i.e., a TS of 1,180 MPa or more, and improved workability such as elongation, stretch flangeability, and bendability. Patent Literature 2 discloses a technique for a high-strength hot-dip galvanized steel sheet in the form of a steel strip with small non-uniformity of strength, excellent in formability, and having high strength, i.e., a TS of 780 MPa or more.